The present invention relates to a sealable transparent multilayer film comprising a base layer formed of a propylene polymer and at least one sealable layer.
German Offenlegungsschrift No. 16 94 694 (equivalent to British Pat. No. 1,145,199) discloses heat-sealable laminates which comprise an oriented polypropylene film having at least one heat-sealable layer formed of an ethylene-propylene copolymer comprising from 2 to 6% by weight of ethylene and from 98 to 94% by weight of propylene. These films exhibit good heat-sealability; however, their scratch-resistance and clearness are not of the desired degree, and, in addition, their processing characteristics in high-speed packaging machines are insufficient.
European Pat. No. 27 586 (equivalent to U.S. Pat. No. 4,348,457) describes sealable polypropylene films, which possess a sealing layer comprising an ethylene bromo- or copolymer and contain a long-chain aliphatic amine, an incompatible thermoplastic component and a polydialkyl siloxane. Although these films are improved over those of British Pat. No. 1,145,199, their reliability of running in horizontal forming, filling and sealing machines is still insufficient.
German Offenlegungsschrift No. 29 41 140 (equivalent to U.S. Pat. No. 4,293,608) discloses a packaging material which comprises a base layer of a polypropylene polymer and a surface layer formed of a blend of a propylene-ethylene copolymer and a C.sub.4 to C.sub.10 alpha-olefin-propylene copolymer. This surface layer may also contain a low-molecular weight thermoplastic resin and silicone oils. Packaging materials of this kind have the disadvantage that they are sensitive to scratching and still have insufficient optical properties.
There has also been proposed a film which possesses sealing layers of specifically composed polyolefinic copolymers or terpolymers. Due to being modified with a propylene homopolymer, a dimethyl polysiloxane and a synthetic resin, this film exhibits good processing properties on high-speed packaging machines, an improved visual appearance and a high scratch resistance. These films fulfill the requirements to be met by a packaging film from the point of view of machine engineering, but handling of the film is rather problematic because of its extremely smooth surfaces which have a very low coefficient of friction. For example, when the film roll having the original full width (about 4 m to 6 m) is cut into rolls of smaller widths, an increased lateral deviation of the cutting edges results, i.e., it is not possible to obtain rolls having exactly the desired width. Instead, this width is exceeded to a greater or lesser amount due to the fact that the film escapes laterally when high cutting speeds are employed. In the description which follows, the steps of slitting and winding-up shall be referred to as "converting".
Yet another disadvantage of the above film resides in its high tendency to telescope on the roll during handling, which may already occur on the slitter or when the film roll is inserted into the packaging machine. Telescoping means a lateral displacement of the film on the roll, in which the surfaces of adjacent layers all slide to one side, as a result of which the edges of the film are no longer positioned exactly above one another. Instead, each layer is displaced in one direction by a small amount, and therefore the whole roll protrudes from one side of the mandrel. Telescoping usually occurs when the rolls are bumped against something or when they are held in a slightly inclined position.
Due to these difficulties, the machine speed must be reduced down to about half the normal speed during converting, and any further handling of the rolls must be done very carefully.